This invention relates to methods of predicting the occurrence of seismic events. More specifically, this invention relates to a method of predicting coastal earthquakes by using ocean acoustic sensors. The coastal area can extend several hundred kilometers inland.
Ocean-based acoustic sensors have been used to study ocean explosions, volcanic eruptions, and earthquakes. Acoustic sensors have been found to be more sensitive than seismic sensors for detection of earth tremors. Acoustic sensors can distinguish three seismic waves, named in accordance with observed transmission speeds through water. Primary waves (P-waves) are caused by longitudinal compression forces and can travel through both solids and liquids. P-Wave acoustic and seismic signals are correlated. Secondary waves (S-waves) are caused by transverse shearing forces and can travel only through elastic rock. S-waves can only be observed indirectly by acoustic sensors when vertical motion is converted to acoustic energy near bottom-mounted sensors. Tertiary waves (T-waves) are caused by the release of sound energy resulting from seismic activity into water. T-waves are rapidly attenuated in land but can travel long distances by horizontal propagation in water. Due to variations in signal amplitude with ocean, depth and topography, signal duration has been shown to be a more reliable indicator of seismic strength.
Seismically active belts exist along coastal areas surrounding the Pacific basin that otherwise shows relative calm, with the exception of a few island groups such as Hawaii. Sources of T-waves have been traced to these seismically active areas and to deep offshore trenches. It is not clear how T-waves are produced, but it is believed that they result from a seismic-to-acoustic coupling on the seafloor. For continental earthquakes, T-waves can be modeled accurately as individual point sources distributed over the ocean floor. Simulation studies indicate that T-waves are excited most efficiently in shallow water within a few hundred kilometers of an earthquake epicenter. Empirical studies have shown that many small seismic events produce P- and S-waves, but not T-waves.
Today, earthquakes are monitored with land-based seismic sensors that do not facilitate earthquake prediction. Acoustic sensors have been found to detect energy changes several hours prior to an earthquake and can be used for prediction.
The invention provides a method of predicting impending coastal seismic activity by using ocean-based acoustic sensors. Acoustic sensors are used to sense and record low frequency acoustic P-waves and T-waves associated with seismic activity. Wave characteristics including frequency, amplitude, and duration of the acoustic events are analyzed to determine the strength and rate of occurrence of T-waves. A decrease in the rate of the occurrence of T-waves during a portion of the selected time period is indicative of impending seismic activity. The unique aspect of this invention is the a priori processing of acoustic (vice seismic) data from ocean-based sensors to predict coastal earthquakes. The scope of this invention is not limited to particular signal processing technique, but includes all techniques capable of characterizing T-waves for predictive purposes.